Data Synchronization in a Cooperative Distributed Control System

ABSTRACT

A data synchronization method and apparatus are provided for a distributed control system where multiple controllers can control a plurality of actuators or sensors. The actuators or sensors are assigned to logical groups that share a common state. One of the actuators or sensors within a logical group serves as a reference device for a state variable. Before taking a control action, the controllers read the current value of the state variable from the reference device. Data synchronization is maintained by one or more refresh master controllers.

BACKGROUND

The present invention relates generally to control systems for processcontrol, and more particularly, to data synchronization in a cooperativedistributed control system where a controlled device, such as actuatoror sensor, is controlled by multiple controllers.

A typical feedback control system comprises a single, centralizedcontroller that receives feedback from one or more sensors and generatescontrol signals for one or more actuators in the process. In this typeof centralized control system, the control logic is implemented by asingle controller with sufficient processing and memory to execute allof the data acquisition and control algorithms for the entire system.

A centralized control system can be easy to create and maintain becauseall of the control logic is at a single location. However, theprocessing requirements can be prohibitive if the central controller isexpected to implement complex control algorithms, or to execute multiplecontrol algorithms in parallel. Similarly, the bandwidth requirements toimplement complex control algorithms may be prohibitive where data needsto be acquired from many remote sensors. Another drawback is thatcentralized control systems lack inherent robustness due to possiblefailure of the centralized controller. To ensure robustness, backupcontrollers may be required to ensure redundancy, which can makecentralized control systems expensive to implement.

A distributed control system is a more complex control system wheremultiple controllers share responsibility for certain control functions.In a distributed control system, there is no single master controllerfor all control functions, and multiple controllers can exercise controlover the same actuator and/or sensor. One example of this type ofcontrol system is a residential heating system with multiple heatingelements and multiple controllers. The heating elements may be logicallygrouped into multiple zones. Each zone is independently controlled tomaintain the temperature within the zone at a desired temperature setpoint. Each of the controllers is able to set the operating mode anddesired temperature for each zone based on user input. In this example,the controllers need to work together in order to set a single mode anddesired temperature for each zone without contention.

Distributed control systems can be scaled more easily than centralizedcontrol systems by adding additional controllers to extend thefunctionality of the control system. Another advantage of distributedcontrol systems is their inherent redundancy. If one controller fails,other controllers are still available to perform the control functionsof the failed controller.

In a distributed control system, each controller needs to know the stateof the actuator or sensor it controls in order to perform the requiredcalculations for its control function. The controller also needs to beable to set the value of state variables for the actuators and sensors.As an example, a user interface in a residential heating system needs toknow the current temperature set point in a zone in order to determinewhether heating or cooling should be applied. As another example, thecontroller may need to know the current temperature set point in orderto determine whether the current temperature setting needs to bechanged.

Because multiple controllers may be able to change a specific statevariable of the actuator or sensor, no controller can assume the currentstate from the last command it sent to the actuator or sensor. Eachactuator or sensor “owns” its current state, and the controller readsthe state of the actuator or sensor each time the controller needs thedata to perform a control action. In a large system with manycontrollers, actuators, and sensors, sufficient communication resourcesneed to be provided to avoid congestion, long latency periods, and datathroughput issues.

In some control systems, the actuators and sensors may be logicallygrouped together because they share a common state, such as an operatingmode or set point. An inconsistency can cause control system errors andambiguous control actions. As an example, in a residential HVAC system,the heating and cooling elements within a zone should have the sametemperature set point. If the heating elements have a higher temperatureset point than the cooling elements, the control system would try toheat and cool the zone simultaneously, with wasteful and ambiguousresults. Thus, inconsistency can cause devices within a logical group towork against each other.

An inconsistent state for actuators and sensors within a group can becaused by a communication failure, a device failure, or interlacedcommands from multiple controllers to members of the logical group.Because there are multiple controllers, more than one controller couldtry to change the state of an actuator or sensor at the same time,causing an inconsistency within the group. Inconsistent states can alsooccur if the number of actuators or sensors within a group change. Anactuator or sensor within a group may be temporarily removed before anew state command is set by a controller and then added back after thecommand has already been sent. An example of this scenario would be whenan actuator or sensor is removed to change the battery. Also, anewly-added actuator or sensor may have a different state than otheractuators and sensors within a logical group.

Accordingly, there is a need to maintain data synchronization betweenactuators and sensors in a logical group that share a common state wherethe state is subject to control by multiple controllers.

SUMMARY

The present invention provides methods and apparatus for datasynchronization in a distributed control system where a controlleddevice, such as actuator or sensor, is controlled by multiplecontrollers. The actuators or sensors are assigned to logical groupsthat share a common state. According to a first aspect of the invention,one of the actuators or sensors within a logical group serves as areference device for a state variable. Before taking a control action,the controllers read the current value of the state variable from thereference device. The assignment of a reference device for the logicalgroup avoids the need to read the value of the state variable from eachactuator or sensor within the logical group, thus reducing the signalingoverhead. When updating a state variable, the controller sends a commandto update the state variable to every actuator and sensor in the logicalgroup.

According to a second aspect of the invention, one of the controllers isdesignated to serve as a refresh master controller for one or more statevariables. The refresh master controller is configured to periodicallysend a refresh command to each actuator and sensor in the logical groupto resynchronize the states of the actuators and sensors. The refreshmaster controller resolves inconsistencies in the states of theactuators and sensors that may arise during normal operation.

In some embodiments of the invention, the identity of the refresh mastercontroller is stored in memory of the reference device. Before sending arefresh command, a controller queries the reference device to confirmthat it is the refresh master controller.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a cooperative distributed control system withmultiple controllers to control the functions of one or more actuatorsand sensors.

FIG. 2 illustrates a reference device for a logical group of actuatorsand sensors.

FIG. 3 illustrates a refresh master controller for a logical groupresponsible for data synchronization.

FIG. 4 illustrates an exemplary signaling flow for updating a statevariable for a logical group of actuators and sensors.

FIG. 5 illustrates an exemplary signaling flow for refreshing a statevariable for a logical group of actuators and sensors.

FIG. 6 illustrates an exemplary data synchronization method for acontrol system where multiple controllers exercise control in acoordinated manner over a logical group of actuators and sensors.

FIG. 7 illustrates an exemplary method of selecting a refresh mastercontroller responsible for data synchronization.

FIG. 8 illustrates an exemplary controller configured to implement thedata synchronization methods herein described.

FIG. 9 illustrates an exemplary actuator in a logical group.

FIG. 10 illustrates an exemplary sensor in a logical group.

DETAILED DESCRIPTION

Referring now to the drawings, FIG. 1 schematically illustrates acooperative distributed control system indicated generally by thenumeral 10. The cooperative distributed control system 10 includes aplurality of controllers 100, a plurality of actuators 200, and aplurality of sensors 300. The controllers 100 share responsibility forcontrolling a process 50. The sensors 300 provide feedback to thecontrollers 100 regarding the current state of the process 50. Inresponse to the feedback from the sensors 300, the controllers 100 cangenerate and send control commands to one or more of the actuators 200to implement a control algorithm. Typically, the commands are used toset the value of state variables for the actuators 200 and sensors 300.The commands typically include an identifier identifying the controllersending the command, a priority indicator indicating the priority of thecontroller, a state variable identifier indicating the state variabletargeted by the command, and a value for the state variable.

In the cooperative distributed control system 10, each controller 100acts independently, but coordinates its actions with other controllers100 to implement the system control functions. Some controllers 100 maybe capable of performing all control functions, while other controllers100 may perform only a subset of the control functions. Thus, some ofthe control functions may be subject to control by more than onecontroller 100. Because more than one controller 100 may be responsiblefor some control functions, there will be command contention and acommand arbitration procedure is needed. U.S. patent application Ser.No. ______ filed ______ describes a command arbitration scheme for acooperative distributed control system. This application is incorporatedherein in its entirety by reference.

The actuators 200 and sensors 300 may be assigned to logical groups 400(FIG. 2) to implement the control functions. The actuators 200 andsensors 300 may be assigned to logical groups 400 responsive to userinput at the time of system installation, or after the systeminstallation. As one example, the control system 10 may control aresidential HVAC system with multiple zones. Each zone includes heatingand air conditioning units to heat and cool the zone respectively. Theactuators 200 and sensors 300 for the zone are assigned to a logicalgroup 400 that shares a common temperature set point and mode (e.g.heating or cooling). Each of the controllers 100 is able to set the modeand desired temperature set point for each zone based on user input.

An actuator 200 or sensor 300 may belong to multiple logical groups 400as shown in Table 1 below. In the example of Table 1, Actuator/Sensor 1is assigned to Groups 1 and 3, Actuator/Sensor 2 is assigned to Groups 2and 3, Actuator/Sensor 3 is assigned to Groups 1 and 4, andActuator/Sensor 4 is assigned to Groups 2 and 4. The common statevariable for groups 1 and 2 is Variable A. The common Variable forGroups 3 and 4 is Variable B. The notation “Actuator/Sensor” indicatesthat the item in question could be either an actuator 200 or a sensor300.

TABLE 1 Group Assignments Actuator/Sensor Group ID State Variable 1 1 A2 2 A 3 1 A 4 2 A 1 3 B 2 3 B 3 4 B 4 4 B

For purposes of maintaining data synchronization, an actuator 200 orsensor 300 within each logical group 400 is selected to serve as areference device for the logical group 400 for a specific statevariable. FIG. 2 illustrates a logical group 400 includingActuator/Sensor 1, Actuator/Sensor 2, and Actuator/Sensor 3.Actuator/Sensor 1 serves as a reference device. The state of eachactuator 200 and sensor 300 within the logical group 400 is consideredto be the same as the state of the reference device. When one of thecontrollers 100 needs to perform a control action, the controller 100reads the current value of the state variable from the reference device.Thus, a single read operation is performed to determine the state of allmembers of the logical group 400. Using a reference device avoids theneed of the controller 100 to read the state of every actuator 200 andsensor 300 within the logical group 400, and reduces the signalingoverhead for the control system 10.

The reference device for each logical group 400 must be uniquelyidentifiable and known to all of the controllers 100. Because eachcomponent in a control system 10 must have a unique network address, theactuator 200 or sensor 300 with the lowest network address can be chosento serve as the reference device for the logical group 400.

In order to ensure that the state data for all of the actuators 200 andsensors 300 within the same logical group 400 are consistent, one of thecontrollers 100 is selected to serve as a refresh master controller.FIG. 3 illustrates a refresh master controller performing a refreshoperation. The refresh master controller periodically refreshes orrewrites the current state of each actuator 200 and sensor 300 within alogical group 40 for which it is responsible to maintain datasynchronization. Refreshing the state of each actuator 200 and sensor300 within a logical group 400 resolves consistency problems that mayoccur during normal operation. For example, a newly added actuator 200or sensor 300 may have a different state than other actuators 200 andsensors 300 within the same logical group 400. Also, an actuator 200 orsensor 300 within a logical group 400 that is temporarily removed, e.g.,to change a battery, may have a different state than other group membersdue to a change of state while the actuator 200 or sensor 300 wasoffline. In these cases, the inconsistency is resolved when the refreshmaster controller refreshes the state of the actuators 200 or sensors300 within the logical group 400.

The actuators 200 and sensors 300 within a logical group 400 may havemultiple independent state variables. A different refresh mastercontroller may be selected for each independent state variable. Also,different state variables may have different reference devices. As anexample, in a residential HVAC system, the actuators 200 and sensors 300within a cooling or heating zone may select one refresh mastercontroller for the mode (e.g., heating or cooling), and a differentrefresh master controller for the temperature set point. In oneexemplary embodiment, the refresh master controller is the lastcontroller 100 to update the state for the logical group 400. When acontroller 100 updates a state variable, a Set State command is sent toeach actuator 200 and sensor 300 in the logical group 400. The Set Statecommand specifies a state variable and a new value for the statevariable. For each state variable, the reference device remembers theidentity of the controller 100 that was the last-to-command. Beforeperforming a refresh operation, a controller 100 must verify that it isthe refresh master controller. Thus, the reference device arbitratescontention between the different controllers 100 and determines whichcontroller 100 is the refresh master controller.

FIG. 4 illustrates an update procedure performed by a controller 100 toupdate a state variable for a logical group. Before updating the statevariable, the controller 100 needs to determine the current value of thestate variable for the logical group 400. As previously noted, thecontroller 100 fetches the current value of the state variable from thereference device for the logical group (step a). More particularly, thecontroller 100 sends a request to the actuator 200 or sensor 300. Inresponse to the request, the actuator 200 or sensor 300 returns thecurrent value. The controller 100 then calculates the new value of thestate variable (step b). To maintain data synchronization, thecontroller 100 sends a Set State command to each member of the logicalgroups (step c). The Set State command indicates the state variable thatis the target of the command and the new value of the state variable.Additional information, such as the controller identification may beincluded in the Set State command. When the Set State command isreceived by an actuator 200 or sensor 300, the actuator 200 or sensor300 sets the value of the state variable to the value specified by theSet State command.

FIG. 5 illustrates an exemplary data refresh sequence. Controller 1sends a Set State command to each of the actuators 200 and sensors 300in a logical group 400 to change a state variable (step a). TheActuator/Sensor 1 serves as the reference device for the logical group400. The Actuator/Sensor 1 remembers the identity of controller 1. Aftersending the command, Controller 1 sets a refresh timer (step b). Whenthe refresh timer expires (step c), Controller 1 queries the referencedevice to determine if it is still the reference master for the logicalgroup 400 (step d). The query is needed because another controller 100may have updated the state variable after Controller 1. When thereference device confirms that Controller 1 is the refresh master (e.g.returns a value of REFRESH MASTER=1), Controller 1 sends commands toeach Actuator/Sensor 1, 2, 3 in the logical group 400 to set the valueof the state variable to the value specified by the last command sent byController 1 (step e).

The data synchronization techniques as herein described enable datasynchronization of actuators 200 and sensors 300 in a logical group 400without direct interaction between the controllers 100. By designatingone of the actuators 200 or sensors 300 to serve as a reference device,signaling overhead, congestion, and latency are reduced. The controlsystem 10 can be easily scaled by adding or removing actuators 200,sensors 300, or both, without manual reconfiguration. Additionally,controllers 100 can also be easily added and removed to the system 10.The addition or removal of an actuator 200 or sensor 300 may result inreselection of the reference device for a group 400 by the controllers100.

FIG. 6 illustrates an exemplary method 150 executed by a controller 100to update a state variable for a logical group 400 of actuators 200 andsensors 300. To start the procedure, the controller 100 determines theactuator 200 or sensor 300 that is serving as the reference device for alogical group 400 (block 152). As previously noted, in one example thereference device may comprise the actuator 200 or sensor 300 with thelowest network address. The controller 100 then reads the current valueof the state variable from the reference device (block 154), which maybe used to perform a control action. The controller 100 may thencalculate a new value for the state variable (block 156). When the newvalue is determined, the controller 100 sends a Set State command toeach actuator 200 and sensor 300 in the logical group to update thestate variable (block 158).

FIG. 7 illustrates an exemplary method 250 implemented by an actuator200 or sensor 300 for selecting the refresh master controller. Theactuator 200 or sensor 300 receives commands at different times toupdate a state variable from a plurality of controllers 100 (block 252).The actuator 200 or sensor 300 designates the last controller 100 toupdate the state variable as the refresh master controller (block 254).The actuator 200 or sensor 300 stores the identity of the refresh mastercontroller in memory of the actuator 200 or sensor 300 (block 256).Thereafter, the actuator 200 or sensor 300 supplies the identity of therefresh master controller to a requesting controller 100 that sends arequest to the actuator 200 or sensor 300 for the identity of therefresh master controller (block 258).

FIG. 8 illustrates an exemplary controller 100 for performing datasynchronization as herein described. Controller 100 comprises a controlcircuit 110, memory 120, a user interface 130, and a communicationinterface 140. The control circuit 110 controls the operation of thecontroller as previously described. The control circuit 110 may beimplemented by one or more processors, hardware circuits, firmware, or acombination thereof. The control circuit 110 includes a commandprocessor 112 that is configured to determine the logical groups andcorresponding reference devices, read the current values of the statevariables from the reference device, and send commands to the actuators200 and sensors 300 in a logical group 400 to update or refresh thestate variables as described above.

Memory 120 stores program instructions and data needed by the controlcircuit 110 to perform its functions. Memory 120 also stores a tablewith the logical groups and the corresponding reference devices. Forexample, the table may include the group identifier, and addresses ofthe group members including the reference device. Memory 120 may, forexample, comprise a non-volatile memory device such an electricallyerasable programmable read only memory (EEPROM), flash memory, ormagnetoresistive random access memory (MRAM). A volatile memory device,such a random access memory (RAM), may also be used to store temporarydata.

The user interface 130 comprises one or more user input devices 132 anda display 134 to enable a user to interact with the control system 10.The user input devices 132 may include a keypad or touch screen forentering data and commands. The display 134 may comprise a liquidcrystal display (LCD) or touch screen display, that also functions as auser input device 132.

The communication interface 140 provides a connection to a local areanetwork (LAN) and enables the controller 100 to communicate with theactuators 200 and sensors 300 in the control system 10. If the LANcomprises a wireline network, the communication interface 140 maycomprise an Ethernet interface. Alternatively, the communicationinterface 140 may comprise a WiFi interface, BLUETOOTH interface, ZIGBEEinterface, or other wireless radio interface to connect to a wirelessaccess point (WAP) in the LAN.

FIG. 9 illustrates the main functional elements of an actuator 200. Theactuator 200 comprises a control circuit 210, memory 220, an actuatingdevice 230, and a communication interface 240. The control circuit 210controls the operation of the actuator 200 as previously described. Thecontrol circuit 210 may be implemented by one or more processors,hardware circuits, firmware, or a combination thereof. The controlcircuit 210 includes a command processor 212 to receive and executecommand from the controllers 100. If the actuator 200 serves as areference device, the command processor 212 also determines whichcontroller 100 is the refresh master controller.

Memory 220 stores program instructions and data needed by the controlcircuit 210 to perform its functions. Memory 220 also stores the currentvalues of state variables of the actuator 200. If the actuator 200serves as a reference device, the memory 220 also stores the identity ofthe refresh master controller. Memory 220 may, for example, comprise anon-volatile memory device such an electrically erasable programmableread only memory (EEPROM), flash memory, or magnetoresistive randomaccess memory (MRAM). A volatile memory device, such a random accessmemory (RAM), may also be used to store temporary data.

The actuating device 230 comprises a control device that is used tocontrol a process. As one example, the actuating device 230 may comprisean on/off switch to control the power to a component in a process. In aresidential heating system, an on/off switch can be used to control thesupply of power to heating elements, fans, or other devices in theprocess. In this case, the state of the switch may comprise one of thestate variables that are controlled by the controllers 100.

The communication interface 240 provides a connection to a local areanetwork (LAN) and enables the actuator 200 to communicate with thecontrollers 100 in the control system 10. If the LAN comprises awireline network, the communication interface may comprise an Ethernetinterface. Alternatively, the communication interface 240 may comprise aWiFi interface, BLUETOOTH interface, ZIGBEE interface, or other wirelessradio interface to connect to a wireless access point (WAP) in the LAN.

FIG. 10 illustrates the main functional elements of a sensor 300. Thesensor 300 comprises a control circuit 310, memory 320, a sensing device330, and a communication interface 340. The control circuit 310 controlsthe operation of the sensor 300 as previously described. The controlcircuit 310 may be implemented by one or more processors, hardwarecircuits, firmware, or a combination thereof. The control circuit 310includes a command processor 312 to receive and execute command from thecontrollers 100. If the sensor 300 serves as a reference device, thecommand processor 312 for the reference device also determines whichcontroller 100 is the refresh master controller.

Memory 320 stores program instructions and data needed by the controlcircuit 310 to perform its functions. Memory 320 also stores the currentvalues of the state variables for the sensor 300. If the sensor 300serves as a reference device, the memory 320 also stores the identity ofthe refresh master controller. Memory 320 may, for example, comprise anon-volatile memory device such an electrically erasable programmableread only memory (EEPROM), flash memory, or magnetoresistive randomaccess memory (MRAM). A volatile memory device, such a random accessmemory may also be used to store temporary data.

The sensing device 330 comprises any type of transducer that senses aparameter of the controlled process and generates an electrical signalindicative of the current value of the parameter. As one example, thesensing device 330 may comprise a temperature transducer to measure thetemperature. In a residential heating system, a temperature transducercan be used to measure the temperature in a room and report thetemperature to the controllers 100.

The communication interface 340 provides a connection to a local areanetwork (LAN) and enables the sensor 300 to communicate with thecontrollers 100 in the control system 10. If the LAN comprises awireline network, the communication interface 340 may comprise anEthernet interface. Alternatively, the communication interface 340 maycomprise a WiFi interface, BLUETOOTH interface, ZIGBEE interface, orother wireless radio interface to connect to a wireless access point(WAP) in the LAN.

The data synchronization methods and procedures described above giveeffective control over a state variable to a master controller withoutdirect interaction between controllers. This data synchronization schemeis advantageous in control systems where controllers are likely to beadded and removed for extensibility and scalability. An example is aresidential heating control system that has a low cost configuration towhich consumers can add additional controllers to extend thefunctionality of the system.

The present invention may, of course, be carried out in other specificways than those herein set forth without departing from the scope andessential characteristics of the invention. The present embodiments are,therefore, to be considered in all respects as illustrative and notrestrictive, and all changes coming within the meaning and equivalencyrange of the appended claims are intended to be embraced therein.

What is claimed is:
 1. A method of updating a state variable implementedby a controller in a distributed control system having multiplecontrollers, said method comprising: determining a reference device fora logical group of actuators and sensors; reading a current value of astate variable from the reference device in the logical group to performa control action; calculating a new value for the state variable; andsending a command to each actuator and sensor in the logical group toset the state variable to the new value.
 2. The method of claim 1further comprising periodically sending a refresh command to eachactuator and sensor in the logical group to set the state variable to apredetermined state.
 3. The method of claim 2 further comprisingverifying that the controller is designated as a refresh mastercontroller for the state variable before sending the refresh command. 4.The method of claim 3 wherein verifying that the controller is a refreshmaster controller for the state variable comprises sending a requestmessage to the reference device and receiving a response message fromthe reference device indicating whether the controller is the refreshmaster controller.
 5. The method of claim 1 wherein determining areference device for a logical group comprises identifying the actuatoror sensor with the lowest network address as the reference device.
 6. Acontroller in a distributed control system, said controller comprising:a communications interface for communicating over a network with aplurality of distributed actuators and sensors in a logical group; and acommand processor configured to: determine a reference device for alogical group of actuators and sensors; read a current value of a statevariable from the reference device in the logical group to perform acontrol action; calculate a new value for the state variable; and send acommand to each actuator and sensor in the logical group to set thestate variable to the new value.
 7. The controller of claim 6 whereinthe command processor is further configured to periodically send arefresh command to each actuator and sensor in the logical group to setthe state variable to a predetermined value.
 8. A method of datasynchronization implemented by an actuator or sensor in a distributedcontrol system having multiple controllers, said method comprising:receiving commands to update a first state variable from two or morecontrollers; designating the last controller to update the first statevariable as a refresh master controller; storing the identity of therefresh master controller for the first state variable in a memory ofthe actuator or sensor; and supplying the identity of the refresh mastercontroller for the first state variable responsive to a query from acontroller.
 9. The method of claim 8 further comprising: receivingcommands to update a second state variable from the two or morecontrollers; designating the last controller to update the second statevariable as a refresh master controller; storing the identity of therefresh master controller for the second state variable in a memory ofthe actuator or sensor; and supplying the identity of the refresh mastercontroller for the second state variable responsive to a query from oneof the two or more controllers.
 10. An actuator or sensor in adistributed control system, said actuator or sensor comprising: acommunications interface for communicating over a network with aplurality of distributed controllers; and a control circuit forarbitrating contention between commands received from differentcontrollers, said control circuit including a command processorconfigured to: receive commands to update a first state variable fromtwo or more controllers; designate the last controller to update thefirst state variable as a refresh master controller; store the identityof the refresh master controller for the first state variable in amemory of the actuator or sensor; and supply the identity of the refreshmaster controller for the first state variable responsive to a queryfrom a controller.
 11. The actuator or sensor of claim 10 wherein thecommand processor is further configured to: receive commands to update asecond state variable from the two or more controllers; designate thelast controller to update the second state variable as a refresh mastercontroller; store the identity of the refresh master controller for thesecond state variable in a memory of the actuator or sensor; and supplythe identity of the refresh master controller for the second statevariable responsive to a query from one of the two or more controllers.12. A method of data synchronization in a distributed control systemhaving multiple controllers, said method comprising: assigning a set ofactuators and sensors to a logical group having one or more shared statevariables; selecting one of the actuators and sensors to serve as areference device for the logical group of actuators and sensors;determining a current state for any of the actuators or sensors withinthe logical group by reading the current value of the state variablefrom the reference device.
 13. The method of claim 12 further comprisingdesignating a selected one of the controllers as a refresh mastercontroller for the state variables, and storing an identity of therefresh master controller in memory of the reference device.
 14. Themethod of claim 13 further comprising periodically sending a refreshcommand from the refresh master controller to each of the actuators andsensors in the logical group to synchronize the states of the actuatorsand sensors.
 15. A distributed control system comprising: a plurality ofactuators and sensors assigned to a logical group that shares one ormore common state variables; and a plurality of distributed controllersconfigured to: select one of the actuators and sensors to serve as areference device for the logical group of actuators and sensors; anddetermine a current state for any of the actuators or sensors within thelogical group by reading the current value of the state variable fromthe reference device.